Pythia 8Philip Ilten

on behalf of the Pythia 8 Team

Massachusetts Institute of Technology

July 20, 2016

10th MC4BSM, Beijing

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Introduction

History

• what is Pythia?A program for the generation of high-energy physics events, i.e.for the description of collisions at high energies betweenelementary particles . . .

• 1978: JetSet from the Lund theory group• 1997: merged into Fortran based Pythia 6• 2004: rewrite into C++ began• 2007: first release of C++ based Pythia 8.1• 2014: mature Pythia 8.2 released

• and its name?Apollon founded the Pythic Oracle in Delphi . . .Questions were tobe put to the Pythia . . .The history of the Pythia program isneither as long nor as dignified as that of its eponym.

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Introduction

Documentation

• An Introduction to PYTHIA 8.2Comput. Phys. Commun. 191, 159 (2015)

• PYTHIA 6.4 Physics and ManualJHEP 0605, 026 (2006)

• any of the original research cited from the manuals

• Pythia 8 online manual (also provided with source)http://home.thep.lu.se/~torbjorn/pythia82html/Welcome.html

• Doxygen documentation (also via help in Python)http://home.thep.lu.se/~torbjorn/doxygen/annotated.html

• mainXY.cc and mainXY.py in examples directory

• email the author list or the authors directlyhttp://home.thep.lu.se/~torbjorn/pythiaaux/contact.html

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Introduction

Getting Started

• download and untar the Pythia 8 sourcew g e t h t t p : / / h o m e . t h e p . lu . se /~ t o r b j o r n / p y t h i a 8 / p y t h i a 8 2 1 9 . t g zt a r −x z v f p y t h i a 8 2 1 9 . t g zcd p y t h i a 8 2 1 9

• configure and compile the source (just requires C++ compiler). / c o n f i g u r em a k e

• build and run an examplecd e x a m p l e sm a k e m a i n 0 1. / m a i n 0 1

// Example main program .# i n c l u d e " P y t h i a 8 / P y t h i a . h "v o i d m a i n ( )

// I n i t i a l i z e .P y t h i a 8 : : P y t h i a p y t h i a ;p y t h i a . r e a d S t r i n g ( " H i g g s S M : a l l = on " ) ;p y t h i a . i n i t ( ) ;p y t h i a . n e x t ( ) ;

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Introduction

Event Anatomy1) hard process

2) resonance decays

3) ISR

4) FSR

5) underlying event

6) hadronization

7) particle decays

W−π−

π0

H

W+

p

p

π+

ντ

π+

ντ

µ−

νµ

π+

γ

γ

τ+

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Internal Hard Processes

Internal Hard Processes

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Internal Hard Processes

Process Selection-- QCD-- Electroweak-- Onia-- Top-- Fourth Generation-- Higgs-- SUSY-- New Gauge Bosons-- Left-Right Symmetry-- Leptoquark-- Compositeness-- Hidden Valleys-- Extra DimensionsA Second Hard ProcessPhase Space CutsCouplings and ScalesStandard-Model ParametersTotal Cross SectionsResonance DecaysTimelike ShowersSpacelike ShowersAutomated Shower VariationsWeak ShowersMultiparton InteractionsBeam RemnantsColour ReconnectionDiffractionFragmentationFlavour SelectionParticle DecaysR-hadrons

1) hard process

2) resonance decays

3) ISR

4) FSR

5) underlying event

6) hadronization

7) particle decays

W−π−

π0

H W+

p

p

π+

ντ

π+

ντ

µ−

νµ

π+

γ

γ

τ+

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Internal Hard Processes

IntroductionLQED = iψ†γµ∂µψ −mψ†ψ − iQgeAµψ†γµψ

− (∂µAν − ∂νAµ) (∂µAν − ∂νAµ)

• build matrix element from diagrams

γ

e−

e+

τ+

τ−

M = ve+iQgeγµue−

−igµνq2 uτ−iQgeγ

νvτ+

• integrate over phase-space for partonic cross-section

σ =∫ ( 1

8π

)2⟨|M|2

⟩(Ee− + Ee+ )

∣∣~pµ− ∣∣|~pe− |

dΩ

• convolute with PDFs for full cross-section

σa1a2→B =∫ ∫

xa1

(xp1 ,Q2, p1

)xa2

(xp2 ,Q2, p2

)σp1p2→B dxp1 dxp2

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Internal Hard Processes

SUSY [arXiv:1109.5852]

• MSSM and nMSSM implementations of SUSY production• all MSSM cross-sections validated• qq → χ0χ0, qq → χ±χ0, qq → χ+χ−

• qg → χ0q, qg → χ±q• gg → gg, qq → gg• qg → qg• gg → q ¯q, qq → q ¯q, qq → qq• qq → ¯q

• q ¯q and qq processes include EW contributions• qq2squarksquark:onlyQCD and qq2squarkantisquark:onlyQCD

• possible to turn on all SUSY production and select requested finalstate(s)

• SUSY:idA, SUSY:idB, SUSY:idVecA, Susy:idVecB

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Internal Hard Processes

SUSY [arXiv:1109.5852]

• super-CKM basis used to describe the mass eigenstates• Ru : (uL, cL, tL, uR, cR, tR)→ (u1, u2, u3, u4, u5, u6)• Rd : (dL, sL, bL, dR, sR, bR)→ (d1, d2, d3, d4, d5, d6)• N : (iB,−iW3,H1,H2)→ (χ0

1, χ02, χ

03, χ

04)

• U : (iW+,H+)→ (χ+1 , χ

+2 )

• V : (iW−,H−)→ (χ−1 , χ−2 )

• couplings and masses must be provided with an SLHA spectrum• from LHEF 〈slha〉 blocks: SLHA:readFrom = 1• directly from SLHA file: SLHA:file = spectrum.slha• either SLHA 1 or 2 allowed, but SLHA 2 preferred• fine-grained options on which parameters taken from SLHA

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Internal Hard Processes

Higgs

• SM Higgs production• f f → H , gg → H , γγ → H• f f → HZ , f f → HW±

• f f → Hf f , qq → Htt/bb, gg → Htt/bb• qg → Hq, gg → Hg, qq → Hg• partial widths can be scaled to NLO: HiggsSM:NLOWidths

• BSM Higgs production• generic two Higgs doublet model, Hu and Hd• five Higgs bosons defined: h0/H 0

1 (H1), H 0/H 02 (H2), A0/H 0

3 (A3), H±• f f → H±, bg → H±t• f f → A0h0, f f → A0H 0, f f → H±h0, f f → H±H 0, f f → H+H−

• standard MSSM CP-even/CP-odd and mass ordering not required• couplings can be read from SLHA spectrum• can also be set via individual coupling parameters:

HiggsH1:coup2d, H1:coup2u, . . .

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Internal Hard Processes

More BSM

• new gauge boson production• f f → Z ′, f f →W ′±

• fully flexible γ/Z/Z ′ interference: Zprime:gmZmode• non-universal couplings allowed: Zprime:vd, Zprime:ad, . . .

• left-right symmetries• includes a SU (2)R group at a larger scale from the SM SU (2)L• based on the model of Nucl. Phys. B 487, 27 (1997)• includes ZR, W±

R , and H++/−−L production

• leptoquarks• simple scalar leptoquark model with arbitrary quark/lepton flavor• q`→ LQ, qg → LQ `, gg → LQ LQ, qq → LQ LQ• default ue− LQ-numbers can be changed: 42:0:products = Q L• required to decay before fragmentation, cannot be stable• cross-section and width modified by k-factor: LeptoQuark:kCoup

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Internal Hard Processes

Even More BSM

• compositeness• composite fermions can result in excited sharp resonances• 2→ 1 processes via gauge boson interactions: dg → d∗, . . .• 2→ 2 processes via contact interactions: qq → d∗q, . . .• decays include matrix element corrections• only gauge boson interaction decays implemented: d∗ → γd

• hidden valleys• based on the work of JHEP 1104, 091 (2011)• hidden unbroken SU (N ) symmetry: HiddenValley:Ngauge• hidden sector mirrors SM fermions: dv, e−v , . . .• gg → qv qv, qq → g → qv qv, f f → γ∗/Z → fv fv• fv can radiate g, γ, and γv/gv

• extra dimensions• Randall-Sundrum resonances based on Phys. Lett. B 503, 341

(2001) (G∗) and JHEP 1201, 018 (2012) (Kaluza-Klein gKK )• γKK and ZKK excited electroweak resonances and unparticle

production

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External Hard Processes

External Hard Processes

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External Hard Processes

Les Houches AccordSUSY Les Houches AccordHepMC InterfaceProMC FilesSemi-Internal ProcessesSemi-Internal ResonancesMadGraph5 ProcessesAlpgen Event InterfaceMatching and Merging-- POWHEG Merging-- aMC@NLO Matching-- CKKW-L Merging-- Jet Matching-- UMEPS Merging-- NLO MergingUser HooksHadron-Level StandaloneExternal DecaysBeam ShapeParton DistributionsJet FindersRandom NumbersImplement New ShowersRIVET usageROOT usageA Python Interface

1) hard process

2) resonance decays

3) ISR

4) FSR

5) underlying event

6) hadronization

7) particle decays

W−π−

π0

H W+

p

p

π+

ντ

π+

ντ

µ−

νµ

π+

γ

γ

τ+

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External Hard Processes

Les Houches Accord

• read in Les Houches Event Format files with versions 1, 2, or 3• set beam input to LHEF: Beams:frameType = 4• provide the LHEF name: Beams:LHEF = events.lhe• optionally provide separate header: Beams:LHEFheader =

header.lhe• full examples provided in main25.cc, main31.cc, main32.cc,

main37.cc, main38.cc, and main43.cc• create an LHAup derived class to pass LHA information to Pythia

• set beam input to LHAup: Beams:frameType = 5• pass LHAup pointer to Pythia instance

p y t h i a . s e t L H A u p P t r ( L H A u p P t r ) ;

• LHAupFortran reads HEPRUP and HEPEUP Fortran common blocks

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External Hard Processes

Semi-Internal Process

• create a SigmaProcess derived class to pass to Pythia• Sigma1Process, Sigma2Process, and Sigma3Process for 2→ 1, 2,

and 3p y t h i a . s e t S i g m a P t r ( S i g m a P r o c e s s P t r ) ;

• example in main22.cc• double SigmaProcess::sigmaHat() calculates the cross-section

• 2→ 1: return σ(s)• 2→ 2: return dσ/dt• 2→ 3: return |M|2 with normalization σ =

∫|M|2 /(2s) dΦ

• string SigmaProcess::inFlux() defines the incoming partons• gg, qg, fgm, ggm, gmgm• qq, qqbar, qqbarSame, ff, ffbarSame, ffbarChg

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External Hard Processes

PowhegBox

• PowhegBox matrix elements, seehttp://powhegbox.mib.infn.it, can be passed via LHAuppointer

• PowhegBox binaries require special compilation flagss e d −i " s / F 7 7 = g f o r t r a n / F 7 7 = g f o r t r a n - r d y n a m i c - f P I E - f P I C - p i e / g→

" M a k e f i l e

• configure Pythia with --with-powheg-bin=/powheg/bin• PowhegProcsIn handles loading the LHAup pointer

• full example given in main33.ccP y t h i a p y t h i a ;P o w h e g P r o c s p r o c s (& p y t h i a , " h v q " ) ;// Read POWHEGBOX c o n f i g u r a t i o n from f i l e .p r o c s . r e a d// Or read from s t r i n g s passed .p r o c s . r e a d S t r i n g ( " i h 1 1 " ) ;p y t h i a . r e a d S t r i n g ( " B e a m s : f r a m e T y p e = 5 " ) ;p r o c s . i n i t ( ) ;p y t h i a . i n i t ( ) ;

• shower matching parameters should be set: POWHEG:*

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External Hard Processes

MadGraph

• two options for using MadGraph with Pythia• output LHEF and pass to Pythia• create SigmaProcess class and pass to Pythia

• first option automated through LHAupMadgraph class• can run with either MG5 or aMC@NLO output• configure Pythia with --with-gzip• creates a GRID-pack structure for fast additional runs• full example given in main34.cc

L H A u p M a d g r a p h m a d g r a p h (& p y t h i a , true , " m a d g r a p h r u n " , " m g 5 _ a M C " ) ;m a d g r a p h . r e a d S t r i n g ( " g e n e r a t e p p > mu + mu - " ) ;m a d g r a p h . r e a d S t r i n g ( " s e t e b e a m 1 6 5 0 0 " ) ;m a d g r a p h . r e a d S t r i n g ( " s e t e b e a m 2 6 5 0 0 " ) ;m a d g r a p h . r e a d S t r i n g ( " s e t m m l l 80 " ) ;p y t h i a . r e a d S t r i n g ( " R a n d o m : s e t S e e d = on " ) ;p y t h i a . r e a d S t r i n g ( " R a n d o m : s e e d = 1 " ) ;p y t h i a . s e t L H A u p P t r (& m a d g r a p h ) ;p y t h i a . i n i t ( ) ;

• remember to set up matching/merging correctly

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Showers

Showers

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Showers

Process Selection-- QCD-- Electroweak-- Onia-- Top-- Fourth Generation-- Higgs-- SUSY-- New Gauge Bosons-- Left-Right Symmetry-- Leptoquark-- Compositeness-- Hidden Valleys-- Extra DimensionsA Second Hard ProcessPhase Space CutsCouplings and ScalesStandard-Model ParametersTotal Cross SectionsResonance DecaysTimelike ShowersSpacelike ShowersAutomated Shower VariationsWeak ShowersMultiparton InteractionsBeam RemnantsColour ReconnectionDiffractionFragmentationFlavour SelectionParticle DecaysR-hadrons

1) hard process

2) resonance decays

3) ISR

4) FSR

5) underlying event

6) hadronization

7) particle decays

W−π−

π0

H W+

p

p

π+

ντ

π+

ντ

µ−

νµ

π+

γ

γ

τ+

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Showers

Introduction

γ/Z

e−

e+

q

q

z = Eg/Eq

θq

θqq

q

g

γ/Z

e−

e+

q

q

γ/Z

e−

e+

q

q

• diverges for three scenarios• z → 0 (soft)• θ → 0 (collinear to q)• θ → π (collinear to q)

• factorize collinear divergences as independent emissions

dσe+e−→qqg ≈ σe+e−→qq

∑i

((dθ2

pi

θ2pi

)(αs

2π

)(N2c − 12Nc

)(1 + (1− z)2

z

)dz)

• generalize for all processes with splitting functions Pbjbi

dσA→Bbj ≈ σA→B

∑i

((dθ2

bi

θ2bi

)Pbj bi (z, αs) dz

)• parton bi emits parton bj

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Showers

Internal Showers

• timelike shower (final state radiation) is fully interleavedpT-ordered

∆ij(q21 , q2

2) = exp

(−∫ q2

1

q22

1q2

∫ 1−Q20/q2

Q20/q2

Pji(z, αs) dz dq2

)• spacelike showers (initial state radiation) is fully interleaved

pT-ordered∆ij(q2

1 , q22 , x) = exp

(−∫ q2

1

q22

1q2

∫ 1−Q20/q2

Q20/q2

Pij(z, αs)

( xzx

)( f (x/z, q2, j)f (x, q2, i)

)dz dq2

)• high Q2 and small x to small q2 and large x

• define cut-off Q0: TimeShower:pTmaxMatch,SpaceShower:pTmaxMatch

• 1 - wimpy: factorization scale• 2 - power : half the dipole mass

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Showers

Automatic Shower Variations [arXiv:1605.08352]

• available only for QCD showers• variations on renormalization scale (multiplicative) and

non-singular terms (additive)• variations turned on with: UncertaintyBands:doVariations• specified by: UncertaintyBands:List = name fsr:muRfac=0.5

isr:muRfac=0.5, ...• keywords for variable terms

• fsr:muRfac, isr:muRfac: renormalization scale factor• fsr:cNS, isr:cNS: additive non-singular term• fsr:G2GG:muRfac, fsr:Q2QG:muRfac, fsr:G2QQ:muRfac,

fsr:X2XG:muRfac: finer grain control• accessed via Pythia::info.weight(i)

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Showers

Weak Showers [JHEP 1404, 115 (2014)]

• weak showers are available, with a few caveats• Bloch-Nordsieck violations from W± flavor changing are not

accounted for• γ∗/Z interference is not handled: low masses use γ∗ and high

masses are Z• activated via: TimeShower:weakShower and

SpaceShower:weakShower• specify the allowed splittings: TimeShower:weakShowerMode and

TimeShower:weakShowerMode• 0 W± and Z• 1 only W±

• 1 only Z

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Showers

External Showers

• DiRe (dipole resummation)• Eur. Phys. J. C 75, no. 9, 461 (2015)• careful treatment of collinear enhancements• very modular and extensible• implemented both as Pythia plugin and within Sherpa• available from https://direforpythia.hepforge.org/

• Vincia (Virtual Numerical Collider with Interleaved Antennae)• arXiv:1605.06142• dipole-antenna shower plugin to Pythia• provide 2→ 3 shower kernels• captures both collinear dynamics and soft singularities• available from http://vincia.hepforge.org

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Showers

Matching and Merging• MLM (main89.cc)

• calculate Sudakov factor on all lines• shower, reject emission using factor

• CKKW-L• perform shower and cluster jets• match jets to partons, reject if Np 6= Njets

• Powheg (main31.cc)• pick largest pT emission from NLO normalizedM• evolve shower downwards to pT scale

• Umeps• unitarized matrix element and parton shower merging• tree-level n-leg merging without inclusive cross-section modification

• Unlops• unitarized next-to-leading-order parton shower• n-leg merging at NLO but more generalized

• FxFx• R. Frederix and S. Frixione• merging and matching of aMC@NLO

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Decays

Decays

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Decays

Process Selection-- QCD-- Electroweak-- Onia-- Top-- Fourth Generation-- Higgs-- SUSY-- New Gauge Bosons-- Left-Right Symmetry-- Leptoquark-- Compositeness-- Hidden Valleys-- Extra DimensionsA Second Hard ProcessPhase Space CutsCouplings and ScalesStandard-Model ParametersTotal Cross SectionsResonance DecaysTimelike ShowersSpacelike ShowersAutomated Shower VariationsWeak ShowersMultiparton InteractionsBeam RemnantsColour ReconnectionDiffractionFragmentationFlavour SelectionParticle DecaysR-hadrons

1) hard process

2) resonance decays

3) ISR

4) FSR

5) underlying event

6) hadronization

7) particle decays

W−π−

π0

H W+

p

p

π+

ντ

π+

ντ

µ−

νµ

π+

γ

γ

τ+

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Decays

Introduction

• m-generator• mass generator• two-body decays through

intermediate masses

p0

p123

p4

p2

p1

p5

p1234

p3

p12

dΦ2(q0, q1, q2) =( 1

(2π)222

)δ(q0 − q1 − q2)

d~q1

E1

d~q2

E2

dΦ3(q0, q1, q2, q3) =( 2π

)dΦ2(q0, q12, q3)m12 dm12 dΦ2(q12, q1, q2)

• re-weight byM for the phase-space point• difference between resonance and particle purely technical

• resonances: states with a typical lifetime shorter than thehadronization scale

• particles: states with a lifetime comparable to or longer than thehadronization scale

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Decays

Resonance Decays

• any state with m0 > 20 GeV is resonance by default• light SUSY particles also treated as resonances

• resonance branching fractions modify the relevant cross-section• SUSY resonance decays implemented with weighting

• q → χ, qW /Z , qq, lq• g → qq• χ→ χW /Z , qq, ˜• χ0 → qqq• ˜→ lχ, ˜W /Z

• long-lived g, b, and t can be allowed to hadronize:RHadrons:allow

• unknown resonances decayed with flat phase-space decays• partial width can be forced, or allowed to run with various schemes

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Decays

Particle Decays

• most particle decays are flat phase-space with some exceptions• ω, φ→ π+π−π0

• V → PS PS with V from PS → PS V or PS → γV• Dalitz decay X → Y `+`−

• double Dalitz decay X → `+`−`+`−

• weak decays• B → γX

• τ decays are not flat phase-space and can handle spin effects• spin effects calculated internally• correlations handled for W , Z , W ′ , Z ′ , H , h0, H 0, A0, and H±• lepton-flavor violating gauge boson decays allowed• CP-mixing of the extended Higgs can be specified• external spin information can override internal calculation

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Decays

External Decays

• create a ResonanceWidths derived class to pass to Pythia• does not re-weight the decay• void ResonanceWidth::calcWidth calculates total width• full example given in main22.cc

• create a DecayHandler derived class to pass to Pythia• 1→ n decays specified via bool DecayHandler::decay• chains specified via bool DecayHandler::decayChain• full example given in main17.cc

• external decays via EvtGen, http://evtgen.warwick.ac.uk• plugin class EvtGenDecays applies decays to Pythia event record• allows forced decays and provides event weight• full example given in main48.cc

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Outlook

Outlook

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Outlook

Outlook

• Pythia designed to be simple and easy-to-use, yet flexible• large selection of internal hard processes for fast use

• external hard process from LHEF input, LHAup pointers, orSigmaProcess

• dedicated plugins for PowhegBox and MadGraph• robust shower algorithms

• Hidden Valley showers and weak shower available• alternative DiRe and Vincia shower plugins• exhaustive collection of matching and merging schemes

• spin correlated tau decays and resonance SUSY decays• not mentioned today

• multi-parton interaction framework• Lund string fragmentation model for hadronization

• new Python interface!• questions on anything? please ask!

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